html, body {
height: 100%;
}
var item1 = $('.for-animate1').offset().top - $(window).scrollTop();
swiper.on('slideChange', function () {
if($('.swiper-slide').data('swiper-slide-index') == 1){
$('.textblock').addClass('opac');
}
if($('.swiper-slide').data('swiper-slide-index') == 0 || $('.swiper-slide').data('swiper-slide-index') == 2){
$('.textblock').removeClass('opac');
}
<div class="special-slider-container">
<div class="swiper-container">
<div class="swiper-wrapper">
<div class="swiper-slide card"><img src="images/carousel_1.png" alt=""><h1 class="textblock" style="position: absolute; color:#fff">HELLO WORLD 1</h1></div>
<div class="swiper-slide card"><img src="images/carousel_1.png" alt=""><h1 class="textblock" style="position: absolute; color:#fff">HELLO WORLD 2</h1></div>
<div class="swiper-slide card"><img src="images/carousel_1.png" alt=""><h1 class="textblock" style="position: absolute; color:#fff">HELLO WORLD 3</h1></div>
</div>
<!-- <button class="swiper-button-prev"></button>
<button class="swiper-button-next"></button> -->
</div>
</div>